This invention relates to a machine tool machining method and, more particularly, to a machine tool machining method so adapted that a tool will not contact a workpiece when a pick-feed is performed.
As shown in FIG. 1, the numerically controlled machining of a curved surface is carried out by performing a first machining operation by moving a tool TL at a cutting velocity along a predetermined path PT1 on a workpiece WK in the direction of the arrow, pick feeding the tool from an end point Pe to a starting point Ps on the next machining path PT2 at the conclusion of machining along the above-mentioned path, performing a second machining operation by moving the tool in a cutting feed mode along the machining path PT2 in the direction of the arrow, and thereafter repeating the pick-feed and the first and second machining operations (referred to as back-and-forth cutting) to machine the curved surface. In such numerically controlled machining of a curved surface, machining is carried out while exercising control in such a manner that the central axis (the one-dot chain line in FIG. 1) of the tool TL is directed normal to the workpiece WK or oriented in a direction having a prescribed angle of inclination with respect to the direction of the normal line at all times. Consequently, the machine tool is arranged to rotate the tool while the tool is being moved along orthogonal axes in three dimensions, and to perform machining while the central axis of the tool is, e.g., brought into agreement with the direction of the normal line to the workpiece. NC data specifying the path include position data (position vectors) for specifying the position of the tool nose, and tool central axis direction data (positions along B and C axes or tool central axis vector) for specifying the direction of the tool central axis. Note that the B and C axis are vertical and horizontal axes of rotation.
In the machining of a curved surface by such back-and-forth cutting, the tool nose will strike the workpiece at high speed when the pick-feed is performed, thereby resulting in an erroneous cutting operation or in damage to the tool, unless an appropriate pick-feed path from the machining end point Pe on the first machining path PT1 to the machining starting point Ps on the second machining path PT2 is determined. To this end, when performing a pick-feed in the prior art, pick-feed paths are determined that will not bring the tool nose into contact with the workpiece, and each pick-feed path is programmed as NC data.
However, in the conventional method, a pick-feed path that will not cause a tool to interfere with a workpiece cannot be determined for any and all curved surfaces through a simple technique. As a result, creating the NC data can be a troublesome task. In addition, to insure that the tool will not interfere with the workpiece, with the conventional method the tool retraction stroke is enlarged and, hence, so is the pick-feed stroke. This is disadvantageous in that actual machining time is prolonged.
The foregoing drawbacks become even more pronounced especially when performing pick-feed while rotating the tool in the directions of the B and C axes. The reason is that even when the path of travel of the tool TL of a machine tool having axes of rotation is a straight line LN in three dimensions X, Y and Z, as shown in FIG. 2, the path traversed by the tool nose is unpredictable rather than linear, as indicated by the dashed line, when the tool is rotated in the directions of the B and C axes at the same time that it is moved along the straight line.